Cases of Legionnaires' disease have been categorized as definitive and presumptive. The sensitivity and specificity of antibody titers of > or = 256 and of urinary antigen ratios of > or = 3 were evaluated in 68 patients with "definitive" Legionnaires' disease and in 636 patients with pneumonia who had negative cultures and did not have fourfold rises in titers of antibody to Legionella pneumophila. An acute-phase antibody titer of > or = 256 did not discriminate between cases and noncases (10% vs. 6%; P = .29). The urinary antigen assay gave a positive result in fewer than 1% of noncases but was positive in 55.9% of all cases. This assay was most sensitive (80%) in cases in which L. pneumophila serogroup 1 was isolated. We propose that the case definition for definitive Legionnaires' disease be expanded to include positive urinary antigen assays and that the category of presumptive Legionnaires' disease--based on acute-phase or standing antibody titers of > or = 256 in the nonoutbreak setting--be discarded. The urinary antigen assay will be a valuable tool in the prompt diagnosis of Legionnaires' disease.